I'm no expert on UPSs, but I've picked up a bit over the years - sometimes learning the expensive way (I've got 1/4 ton of dead 12V VRLA batteries to play with now - the boss isn't happy even though I did say from the outset we need to improve the cooling as well
) That and a converter module on the 12kVA UPS going tits-up as well, so we don't even get power conditioning - but that's a different story.
I have the offer of a free 5kVA unit from my old place - if I had room to store it or a use for it.
The first thing I'd suggest checking is whether your larger UPS does actually charge/float the battery from it's input. I'd assumed the DC bus in ours at work was directly connected to the battery but it isn't - there's a separate DC bus internally which is separate from the battery bus. Battery charging is done by a separate module in the power converter blocks, and I think there is a DC-DC converter between battery and internal DC bus. Yes, it sounds more complicated than it needs to be
Hmm, reading that back, it doesn't sound as clear as when I wrote it - but I can't think how to express it better. What I'm trying to say is that I don't think it's necessarily as simple as an AC-DC conversion to the battery bus, a DC-AC conversion off the battery bus, and the load will be supplied by a combination of battery and whatever the input will provide.
That detail is, of course, going to be manufacturer/model specific, so (especially for smaller units) it may be that the internal DC bus is directly connected to the batteries.
The biggest issue is that an online UPS has significant losses all the time. You are doing an AC-DC conversion followed by a DC-AC conversion, both with losses. Overall I think a decent unit should be rated to something in the high 90s % efficient, but it's still going to give you a permanent energy loss. Of course, if you're getting "free" energy then this may well be an acceptable tradeoff - it does give one advantage in that you always get "clean" power out of it regardless of what's going in.
An off-line or line-interactive UPS will have less losses when passing power through - it just sits there watching the power, and the only losses should be for battery charging and it's own power consumption. The downside is that it doesn't clean up the supply current much (they tend to have some filtering, but that's it) as they just pass the AC supply through to the outlet when not running on battery.
But back to your setup. I think you would be correct to say that your load will run from the 300W inverter when it is low enough. The UPS will see a "normal" supply and pass it through (with the aforementioned double conversion). When load increases enough, the inverter will cut out and the UPS will pick up the load - I doubt if there will be any load sharing.
What I suspect will happen is that as soon as the UPS kicks in, the inverter will recover, the UPS will switch back to AC, the inverter will collapse, and so it will cycle. This is one area where the double conversion will be very good - a line-interactive or offline UPS will be clicking like mad as it switches the load between AC and battery, and I hate to think what the load will get fed with.
I have some experience with this.
One of our customers had a scheduled power cut a while ago for some utility engineering works. We were supposed to shift their data and some hosting to our server room so their customers could continue using the online services. Unfortunately that didn't work out, and so "plan C" got invoked - I headed up there with my little petrol genny and an extension lead. Trouble was, the load was probably 80+% of the genny capacity, and things were already on battery when I got there. I fired up the genny, the UPS waited for the supply to settle, then switched over to "mains". The genny output drooped as the load was dumped on it, and so the UPS switched back to battery - repeating each time the genny recovered, I gate to think what got fed to the equipment. Luckily setting the UPS to be less sensitive was enough and we got the whole IT setup running off my genny all day. I was thinking we'd have to shut down several servers (possibly all of them) to reduce load, and then bring them back up one at a time once we'd got the genny online.
From an efficiency and load sharing POV, I think a better arrangement would be a DC-DC converter to transfer power from one battery bank to the other, then an inverter off the second bank. Under light load, the second battery bank will just float and your power path will be 1st battery -> DC-DC -> Inverter -> load. Assuming your DC-DC converter is power limited rather than just trips, under higher load you would indeed share batteries - the inverter drawing from both the DC-DC converter and the second battery. The downside of course is that you are less likely to find the equipment lying around free (or nearly so).
The fundamental problem using old UPSs is that they are designed for "one or t'other" and don't have a facility to share a limited AC supply with battery.
Better still would be running everything off one voltage so you can parallel the batteries - then you just have the one DC-AC conversion.
Hope those rambling make some sense
