The only way to show Opposing Attorneys in the Matter Window is to view them in the detail window for the Parties Tab. Open the Matter, click the Parties Tab, then double-click the non-client party for whom you want to see opposing attorneys. The detail window will list them near the bottom (assuming they've been entered). Since each opposing party may have different attorneys, and there may be more than one, this seems like the best way to show them all.
The only other way to show opposing attorneys would be to add custom fields and double-enter the information, but finding them on the Custom Fields Tab would not be much easier than finding them on the Parties Tab's detail window, and the extra effort to keep the custom fields in sync seems counter-productive.