I have a hole plugged in my basement floor which I'm certain is the toilet drain but don't see anything else, except for 2 pvc pipes jutting then joined halfway toward the first floor right behind this hole. If this is the case, can I piggyback sink and shower drains to the toilet drain. What are those pvc pipes?

It's hard to say without seeing them.

Can you provide a picture's?

I'd feel a lot better if I knew what size the pipes were. The two PVC pipes you speak of; " 2 pvc pipes jutting then joined halfway toward the first floor right behind this hole." In the wall? How far out from the wall is the hole?
Are the two pipes just connected together and terminate or do they tie into something?
Growler was correct when he asked for more details. Can you send us a picture?
If you connect to the toilet drain line the lavatory will have to be vented and, if local codes allow, the shower will connectto the lavatory drain and be wet-vented by it. But all this must be done under the slab unless you're planing on platforming the group. Let us know, Tom