It might be wiser/easier in the long run to utilize a second airport express at the receiver downstairs, considering the AEX has digital audio output (via mini-toslink). The pressing question is, would this suit your particular needs based on input source(s).

The other idea could be to simply add another zone/set of speakers downstairs with a simple speaker selector/secondary amp. Is the second receiver even necessary in this regard?

Other whole home audio options should be considered as well if the party scenario you have brought up here is a frequent issue.