In a "perfect", loss-less combiner, signals in-phase would yield 3 dB gain and
out-of-phase would CANCEL. With unequal signal amplitudes, the output signal
would be a (complex) addition of the inputs.
Stripline (aka Wilkinson) Couplers are very close to this ideal (typ 0.2 to 0.5 dB loss).
The Lindsay UHF Stripline Coupler cost about $100, back when you could still get it...
Various other companies make VHF and UHF Stripline Couplers, but I have yet
to located something "affordable" for the entire UHF band.
However, a cheap RF Spitter/Combiner is more complicated, since it contains
a Hybid Transformer and Mismatch Resistor:
Any phase or amplitude differences between inputs will be lost in the
internal mismatch resistor. With a typical internal loss of 0.3 to 0.5, the
combining gain reaches a max of about 2.5-2.7 dB ONLY when the
two antennas are perfectly matched in GAIN AND PHASE...good luck...
However, stacking antennas ALSO provides the ability for one antenna to maintain
good signal strength while the other suffers a multipath null....which would be
even more effective if a Stripline coupler avoids the Hybrid mismatch loss.
If antennas are pointed in different directions, the loss is closer to 3.3-3.5+ dB
(I've measured up to 4 dB in some splitters).