I can not seem to find an answer to this in the rules.

Several sources have effects which turn critical wounds into normal wounds for a certain period of time.

Cauterize, (Player's Guide 248) is one of them.

If a critical wound has been turned (temporarily) into a normal wound, can the normal wound be healed and the critical wound with it?
I would think that if the "critical" wound is healed in its converted state, there is nothing for it to it to regress to once the stipulated period of time is over and it should be gone completely.

Is there a rule I have missed or must this be a DM ruling?

It would depend on the wording of the card.

For example, (and I'm going off memory), I believe that Cauterize simply allows the recipient to ignore the critical … it doesn't actually convert the critical wound to a normal wound.

If a critical wound actually gets changed into a normal wound, then I would say that the wound could be healed as if it were a normal wound. However, as a GM, I would require/stipulate that that particular wound must be the LAST normal wound healed. Because, otherwise, at the end of the temporary period, a normal wound would need to be reverted back to a critical.

The wording on cauterise is: "That wound is treated as a normal wound until the end of the act."

I do like your suggestion, but I was really hoping there was an actual rule here (my group, including I, are almost all rules lawyers).

No, sorry. There is nothing specific in the rules. WFRP gives the GM a lot of leeway, so a number of rules are loose. As well, this is one of those edge cases that they probably didn't anticipate.

So, it rests on the GM to adjudicate whether he feels he should allow a temporarily "treated as normal" critical wound to be healed before other actual normal wounds. Personally, I'd say no.