I would convert everything to XLR, in this case with a 1/4" to XLR adapter. Note that you didn't say if the 1/4" is TRS or just TS (balanced or un-balanced). TRS is preferable, but either will work. The adapter should be a good quality cable, not one of those off-the-shelf crappy one-piece ones. For combining the signal, I would use a passive, resistive summing box. Someone just described that circuit in another post here not long ago, but in any case, all it is is two female XLR's with Pins 2 & 3 each going through a 3k3 resistor and joining at Pins 2 & 3 on a male XLR. I wouldn't use any of the budget transformer combiners as those transformers can't handle low end that well.
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