This answers to your questions largely depend on what kind of cards you have. In most cases, you should be fine with two GPUs on the same cable. However, I'd avoid using two cards that both require the full 8 pin PCIe connector. You might be okay, but you might not, and I'd err on the side of caution. In general, two cards that only need 6 pin connectors should be fine using the same branch. One 8 pin and one 6 pin should also be fine (in general). As Benarand mentioned, you'll need to check out the max power draw of each of your cards and estimate the max power draw you'd expect to be drawn through the PCIe power connectors or the slot/riser
for your specific cards.
Monitor your rig for a good 30+ minutes after you have all the connections sorted out and start mining. Try to feel if the wires and/or connectors are significantly hotter than the surrounding air. That's a sign that you might not want to run them 24/7 in that configuration. (However, the inverse isn't necessarily true: Just because you can't feel them running hot does not mean that there's no problem.)
For your second question, you'll need to estimate your total power draw based on your specific hardware. Generally, you want your estimated total power draw to be less than about 70% of your power supply's rating. Many are fine when pushed closer to the rating, but err on the side of caution, since most consumer power supplies weren't intended for 24/7 operation. (And blowing a power supply is no fun.) Corsair usually sells quality PSUs, even their budget ones are pretty good, although any PSU from any brand can turn out to be a dud.