Yeah thats why I used the txv for the 1st stage.Just adjust on the fly with super heat control.But remember to adjust superheat after you are running with the 2nd stage refrigerant in the system.The higher pressure affects the operation of the txv and it needs defferant superheat adjustments.

One thing I found on my autocascade is that it really could have benefitted from a cpr valve as pull down made the compressor moan and groan.Static pressure was usually only 200psi.Also start up pressure can really get high, usually about 350 or so.But I have a 2nd stage discharge dump tank solenoid that dumps it to the relief tank which returns to suction via capillary.
Definately plan on using a big relief tank and making it a dump tank would be a good idea too.

Its not like a regular cascade where the only refrigerant in it is the 2nd stage refrigerant at 100psi static.You have to deal with the static from your 1st stage refrigerant combined with what your 2nd stage refrigerant adds.Which could be up to 100psi over 1st stage static if you dont have a big relief tank.