Has anyone any experience of, or information on, holding two retirement visas, from say two SE Asia countries, simultaneously in the same passport?

 

TIA

For the other countries or Thailand?

What do you mean by retirement visa? It could mean a visa issued by a embassy or consulate or wrongly calling an extension stay issued by immigration here a visa.

Sorry. I have an extension of stay based on retirement in Thailand, and am interested in obtaining a 12 month ER extension of stay in Cambodia, to run concurrently.

No problem at all. Many people who use more than one base have something similar. However, these aren't necessarily "retirement" visas as such, rather just the various long stay options offered by the countries in question. As long as you satisfy the financial extension of stay requirements in Thailand they don't care about any concurrent arrangements you might make in other countries. 

That's great, thanks.