back in the '00s i used a hardware kvm that could be controlled by the keyboard with some weird key combo (~ ~ (1|2)? maybe?). these days i strongly prefer deskflow (oss version of synergy) for this sort of thing or just ordinary remote desktop for the secondary. (depends on the task, if you're just building for the secondary or reading email it doesn't really matter- but if you're developing interactive applications or you need to reboot a bunch or something, then having the physical hardware with a local head can help).