Good evening,
You are not specifying if you will use those interfaces on a MAC or a PC. To my knowledge, you won't be able to use 2 focusrite Scarlett series on a PC. On a MAC, you will need to check with Focusrite... since OSX allows audio interface aggregation but with some limitations (I think that USB is not supported...). Ideally check with Focusrite support.
I recall that with ASIO4ALL it was something possible (to use multiple similar audio interfaces... and that was about 10 years ago, not sure now with the new series and OS). Really don't recommend that
Your best solution is to use a 24AO. You will be able to use up to 3 MOTU 24AO on a PC (with a maximum if I recall of 64 channels using USB, not 72). MOTU 24AO are AVB interfaces linked together by an RG45 cable.
Focusrite RED is a DANTE compatible series who would also allow many outputs... but at a cost!.
Unless you audio inputs, why buy an 18i20 to later on switch to a 24AO. Go with a 24AO right away if you know that in the future you will need more outputs.