I notice my combo antennas do NOT have a splitter in joining the two
antennas.

If I add a UHF antenna to my VHF antenna - 4 feet apart, MUST I use a
balum or joiner/splitter 300 to 75 bla bla bla.

What is the result if I just join them with 300 ohm twin lead and
then balum to 75 coax for the run to the TV?

When you connect the ends of two cables together to one feed cable, you get
a different impedance. If that new impedance is what you want to terminate
the feeder with, then you have an impedance match. If it is not a match then
the mismatch will result in a standing-wave back to the antenna. The
standing wave will create an impedance gradient (repeated at 1/2-wave x VF
intervals) along the cable, and that gradient could also be used to match to
a wanted impedance.

If you terminate a 300-Ohms cable from your antenna with a dead-short, then
the characteristic impedance of the cable again will vary along its length,
becoming high/low at 1/4-wave intervals - HLHLHLHLHLHL... etc. In principle
you can connect a 75-Ohm cable at the correct point in the 300 Ohm cable and
get a good match.

An antenna may also have a low impedance within its resonance band, but a
high impedance at other frequencies. Two such antennas can be paralleled so
that power is only transferred to the antenna having a low impedance. This
technique can be used at VHF/UHF/SHF/FHF, etc.

All these techniques can often be compounded with each other to form quite
complex antenna feed solutions.