[NiHaoMike]

Something to beware of is oil return. R290 (either as a refrigerant or an additive) easily solves the oil return issue, but connecting the evaporators in series makes it unlikely to be an issue in the first place.

I have used an ejector approach where I divide up the evaporator into two sections. One part goes where an evaporator would go on a normal system. Between the first evaporator and the TXV, there's an assembly known as an ejector (a kind of pump that has no moving parts apart from the fluid itself) which basically creates a reduced pressure zone for the second evaporator. I also have a phase separator between the first evaporator and ejector tube to supply liquid to the second evaporator. I used a way oversized TXV (5 ton in a 1/2 ton system, can be found cheap on the surplus market) to control flow to the second evaporator. (The reason why the second TXV must be way oversized is because the pressure difference across it is much smaller than in normal use. While a R22 TXV normally sees on the order of 100-200 PSI across it, the TXV for the second evaporator would probably only see 10-20 PSI or so.)

BTW, the ejector approach was first used in the Prius A/C system. By cooling the air in two stages, dehumidification is improved. I highly doubt it would offer much (if any) advantage for a heat pump, and it would certainly be difficult to design such that it would work both ways. Just stick to a series connection.