Obviously, it is inappropriate to allow a 6th-level character to double their full-attacks. That said...
As it turns out, the rules for Pounce are kind of a mess
I am actually going to borrow some terminology from that other Wizards product to describe what’s going on, but it is important to keep in mind that D&D doesn’t use these or define these terms, which is why this is not a determinable case under the rules.
So, in Magic terms, both Pounce and sphinx claws are written like triggered abilities. In Magic, triggered abilities are indicated by phrasing like “when,” “whenever,” or “at,” and both Pounce and sphinx claws are clearly using the “when” phrasing. In fact, they both happen “when” the same event occurs: you complete a charge. Magic has detailed timing rules, and states explicitly that simultaneously-triggered abilities go on the stack in any order their controller wishes.1
However, despite how they are written, this is not how anyone actually runs either ability. I have never met anyone who has treated Pounce as a triggered ability: everyone runs it as what Magic calls a “replacement effect,” specifically a replacement of the single attack you usually get when you charge. The descriptions of neither Pounce nor sphinx claws indicate this, however. As written, as triggered abilities, you would expect the full attack to follow after a charge – including its one attack.
If Pounce and sphinx claws are understood as replacement effects, even though they definitely aren’t written as such, then this becomes clear: both are replacing the same thing, you cannot get both.
If we go with the rules as written, then both are triggered by the conclusion of the charge (including its own solitary attack), and then we run into a conundrum: Magic defines what happens when two abilities are triggered by the same event. Dungeons & Dragons 3.5e does not. Anything and everything is possible, including having to just pick one and not getting the other at all.
The most reasonable approach, I would think, is to mimic the bonus-stacking rules: multiple triggered abilities can all happen, in any order the controller chooses, but you cannot get the same thing twice. In this sense, the “full attack” is effectively a “type” of bonus effect, that does not stack with itself. But then, while this is most reasonable for full attacks, I’m sure there are other cases where this seems less fair.
- Technically, Magic has the players who have abilities triggered by an event each place one of the triggered abilities they control on the stack consecutively, continuing around the players until all triggered abilities are handled.