The easiest thing to do is to use single ended matches on each output that run to connectors and then buy a connectorized wideband balun to make the measurement. This way you can measure each output individually and/or combined through the balun.

I suppose you could synthesize what you want by designing a lumped element circuit that converts the differential signals to in-phase signals then have a network to combine the power.

The differential to in-phase is a phase rotation of +90 and -90 (or +270) degrees. You might be able to use lumped element equivalents to transmission lines to get the desired phase shifts.

If you can achieve the desired phase shift and impedance match then all you need is a power combiner (either external or a lumped element equivalent).

It just depends on how much time you want to put into designing the lumped element circuits and how well the circuits perform when component variation and board parasitics are considered.