Compression notes

At what compression would I have to have my engine if I wanted to twin turbo a B18C5 with a T3 and a T66?

i don't think twin turbocharging a small 4 banger is a good ideea stick with a singe turbo. If you want to get rid of lag get nitrous.
compresionwise i say stay below 9:1

8.5:1 - 8.8:1

Dont waste your money on a B18c5 to turbo it - and second of all dont even bother trying to Twin Turbo a honda motor.

if you want to turbocharge a b18c5 motor do a single turbo setup... and as far as compression ratio the lowest aftermarket pistons I've seen is 9.8 to 1, maybe 9.5 to 1 for the b18c5... so you would have to do some serious machine work to get your compression below a 9.0 to 1... but if you can accomplish a 9.0 to 1 ratio on type r block then you should be fine to run some serious boost provided you have some good internals along with a good resleeving job.