new rims and two flats in one day

I just bought custom built mavic open pros front and rear and changed over the tubes and tires over to the new rims and rode the bike for 5 minutes and everything went smoothly.

The next day, both tires were flat. i've changed flats with no problems before, did i miss something?

Rim tape.

Yes, check/change the rim tape ... VELOX cloth rim tape may still be the most reliable.

Hope I'm not stating what's obvious to you already, but when you check the rim tape, look for spoke ends that are sticking up beyond the nipples. If one or more spokes is too long, that condition needs to be fixed first, before the new tape.

Before pulling off the tyres to fix the flats, mark their position relative to the valve hole. When the puncture is found then the tyre and rim can be inspected in the exact proximity and the cause of the flat determined.

No help if it's pinch flats or careless tyre fitting

A long time ago I read here on this forum to position the 'max pressure' label on the tyre centered on the valve hole. This is a very good habit with many benefits!

Or put the label of the tire at the valve hole, centered.