A question, can I use two PCs to mine in the same account? is this against the rules or something?
Yes, you can, we allow this.
can i use mutiple gpu or more gpu in one unit?
You could, but they should have as similar specs as possible. At the very least, you will need similar VRAM amounts across the cards. If you have a 5GB+ VRAM card as your primary one, then all others should at least have 5GB as well.
Trying to run a 5GB+ VRAM card along with a few 4GB cards should work, however you won’t make as much from your primary (5GB VRAM) card.
Avoid 900-series GPUs for the time being as well, as those haven’t been working with our miners.