Most of these processors can be used concurrently no problem. They just share the same htpasswd file and that causes no probs. If any of the processors need to refresh, they just overwrite the htpasswd file.
I like Globill a lot. I suggest also taking a look at ACPay (
http://www.acpay.com). Although, it's a new service, they're the new service by Cybernet Ventures, which is the Adult Check company, a very reputable company. And they're not in on this $750/$350 extra fee bandwagon.