It won't work that way. (Okay, unfair answer, since this is the "House Rules" forum.) What I mean is, under the Core rules, he'd have to take his 11th level as Cleric, and the BAB etc. progressions would stack (so no add to BAB, +2 to Will saves).
The only thing that comes close to what you're trying to do here is the Gestalt rules in Unearthed Arcana, where PCs take two character classes simultaneously throughout there careers, but the benefits don't stack. For example, a 10th level character who chooses Ranger and Cleric would have the BAB of the Ranger (because it's the better BAB), not the Ranger and Cleric BABs added together. Typically, a gestalt character is about the equivalent of two levels "better" than a regular character, but it isn't advisable to allow gestalt and regular characters in the same campaign, because the increased flexibility (and a bunch of oddities on how things work, like being able to have all good saves) makes them very different.
If you really wanted to allow this as a house rule, I'd say the "second progression" should cost a lot more than the normal starting character XPs, at least three times as much. And make certain that no same abilities stack (so he won't get the BAB or save bonuses from the lesser class, although if he takes enough levels of Cleric, eventually the Will save from the Cleric progression might take the place of the Ranger one).
But all things considered, I wouldn't recommend allowing this at all, because it will get really unbalanced and that can cause some friction between members of a group. I'd strongly recommend you just require him to take single levels of Cleric as per the Core rules instead.
OR, if others in the group are also tempted to take a secondary class, you could consider running a new campaign (or converting this campaign) to have all PCs be gestalt characters. You'll need to review those rules and the relevant recommendations/guidelines, and I strongly recommend, if you do this, having everyone start from 1st level so you can get a feel for the changes in what sorts of things challenge a Gestalt party.