Can I get a landing visa if I already have a visitor visa?

My friend is coming to Taiwan briefly in April, but he needs to visit China in May for a few months, then he’s planning to come here and stay for a while (probably extend the visitor visa to 6 months etc.) So I’m wondering if it’s possible the immigration can give him a landing visa, although his passport has a visitor visa already? I also heard somewhere that you can use the visitor visa multiple times and it resets the 2 month period?

I have one other question unrelated. If we get married, would he be eligible for an ARC through my ARC? I work here and hold an ARC but he would not be working, just living here with my support. How does it work if we marry in Taiwan? Is there a waiting period?

Thanks a bunch, I’m a long-time lurker here, this is a great community!