Definitely doable, we've got both an GSHP / LPG and an ASHP / Oil hybrid to do over the next few weeks.
For ASHP, we'd tend to run it down to 3°C switch over 100% to the fossil burner and then switch back when ot gets up to 6°C - that keeps the ASHP in the sweet spot.
Ideally you need a buffer tank to separate all the flows, you may get away with a low loss header. This way you can keep the existing rads and depending upon outside temp run then at 35-40-45-50 as required if 50 isn't coping then swap over to the LPG. (With a buffer tank you'd run the rads at Δt10 and the heat pump at Δt5)
You also need to think about how you're going to heat the DHW, if you're not going to change the tank (yet) and put in one with a heat pump coil, then run that permanently off the LPG.
Not checked out the Mitsubishi controls manual yet, thought it's really easy to configure the Samsung units to work this way.
Have a look at this from Samsung:
http://www.samsungehs.co.uk/index.php/downloads/finish/6-installation-manuals/159-schematics-showing-what-you-can-do-with-samsung-ehs-monobloc-heat-pumps-bi-valent-hybrid-joining-them-together