How to politely decline a postdoc after the interview, when another postdoc is maybe still in game (same institution)
You should be honest, or at least mostly honest, with them.

Let them know you are also waiting to hear back on your application to another position at the same institution. It's up to you whether or not you say you would favor that position, but I think that is not necessary (hence the "mostly honest") - you can simply approach it as wanting to know all the options available to you before making a decision. This type of request is quite reasonable.

However, there may also be time limits: the group currently offering you a position may want to offer it to another candidate, or at least continue with more interviews. In stating that you would like to delay making a decision, you should also ask about any time limits on acceptance. If they give you some limit, you will then have to decide whether to take the job once that time is expired.