The Mega65 was originally planned to have 2 onboard sockets for SID chips. But since a 6 layer PCB appearantly is going to be too costly with the extra real estate for the sockets the devs opted for 2 SIDs and 2 DACs in the fpga - and relocated the SID sockets to a planned cartridge as for now, afaik.
Since the ArmSiDs is a drop-in replacement for the original chip, it should function in that cartridge too.
I have never seen normal 64's with more than 2 sids onboard - but i wonder how a C64 reloaded mk2 board with 2 sidfx for a total of 4 sids will pan out? But you still need some ribbon cables for that.
You could try to ask the makers of these boards (and the sidfx).
https://icomp.de/shop-icomp/de/shop/product/c64-reloaded-mk2.html
https://ultimate64.com/Ultimate-64-Elite
