That's the same as ours - you just set the flap to "learn" mode, and then the next cat to go up to it and put their head up to the "reader" gets their chip scanned. We had bother with Malt though because bless her heart, no matter how much she tried the flap wouldn't work for her. Turns out her chip had failed, and so we had to get her chipped again. No wonder the gurlies hate us trying to pick them up- they must've been to the vet no fewer than 5 occasions by the time we'd had them 4 months. They went for their initial kitten check, when our vet suggested he leave their vacs until the next visit. Then we had the two vaccination visits, then there was their neutering, and then Malt had to go back for the re-chip.
Lyn, from the sounds of it, the one you would need is the dualscan flap. We have that one on our inner cat flap which leads into the porch, and we have one of the larger pet door ones on the front door (from when we had Ross, who was a beefy boy, bless him.
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We got the dualscan flap when we had Tinks and then got Moray, as we didn't want Moray going out, though Tinks needed to come and go freely.
Funny though - got up this morning to find Barley hissing at the inner flap, and lo and behold - there was Norty Ted, sitting in the no-man's land between flaps, looking a little bemused.
Sometimes, when Barley slinks through the flap (as she does) the outer flap doesn't always close properly, which means it can be pushed open by an enquiring nose. Thankfully, the inner flap has no such problem, hence he couldn't get any further (much to Malt's relief). Sureflap will address the problem if you need it, but thankfully it doesn't happen to us that often, and Ted was mannerly enough to leave when OH opened the door for him.