My opinion is that I would get them both done at the same time, partly for the convenience of one day of fasting, post op recovery etc. but also because it seems that Riley does need a dental, just not as urgently as Lu. If his teeth were fine the dentist would, presumably, have not said he would need a dental in as little as 6 months. If that's the case, I would just go ahead and get it done now rather than hold off a few months if it's going to need doing within the next year anyway.
I think it's hard to know for sure whether they will need extractions, or indeed how bad it is, until they get them under an anaesthetic and can have a good look inside. I had a similar situation with Jaffa when he was 8 - advised at routine annual examination that he would need a dental in the next few months so I booked him in and when they were able to do a proper examination they discovered he did actually have a broken tooth and another that needed taking out. My insurer at the time (petplan) paid out in full for the treatment (minus excess of course).