Yes, you can use a RevECom / Paysystems IBA account on multiple web sites. The "company" name will appear on the Paysystems payment page, though, so it might be best if you have one "blanket" company name that covers all of your sites rather than using one site's name...unless you make it very very clear and open that all of your different sites are related.
It may be more advantageous to use a "blanket account" to support all your sites.
Here are few reasons:
1) Less administrative overhead. You'll have less passwords to remember, and will be able to control all payments and billing from one central location.
2) If your customers see that your company operates several different websites, it lends credibility by implying you're a decent sized business, not some one man in the basement outfit.
3) Use phrases like "our sister company," "owned by Atlanta-based (or whatever city) Sig Online Solutions (or whatever company)." Gives a a more professional and sophisticated perception of your company.