FWIW, I use a pair of OLD Sunpack pro 120 J's, wired to the cam and slaved for those few times when I need studio flash. Of course, it's manual. Sheesh! I also built my reflectors, and my macro set-up. Those who cannot or will not innovate are doomed to mediocrity, and high prices!
The incredibly simple solution is to get a pair of vintage strobes. (Metz 45's are great) Cable one to the cam via hotshoe adapter. Slave the other, via hotshoe adapter. Stop whining, and use your brain. You can't blame the equipment for your failure to make photographs!
I believe you miss the point...
-> It's not about manual flash nor manual slaves since nymphetamine
and I are already proficient at this - just refer to the previous link... for illustration. It's about why Canon doesn't put a Master(Commander) mode in their built-in flash for E-TTL like others!
BTW if you're already in manual for studio setup you might not actually need to cable anything to a hotshoe adapter - Just mask the initial pre-flash from the built-in flash with your hand when pressing the *-button (or FEL). The 2nd strobe that goes with the regular shot will trigger any optical slaves...