Would I be able to use two monoband antennas to a diplexer that's sealed up at the antennas, use a single coax line, then another diplexer at the bottom to the two bands and have minimal losses and adequate if not better performance than a multiband antenna?
Yes, absolutely. Diplexers at both ends have almost zero attenuation. If you instead use a splitter at the receivers you'll loose more than 3 dB.
What kind of signal levels do you expect to have and what are their frequency bands? Is this at home or at a remote hard to get to location that has to be maintenance free and fool proof?
You could probably use RG6 coax and a preamplifier at the common coax at the antennas and then don't need more than one, that could be expensive, diplexer.
If the signal levels are that low that you'll need separate mono banders then the preamp will overcome any coax and splitter losses and any bandpass filters you might need for your receivers. You can then use a 1-4 splitter, or more, and feed other receivers you have.
If using a preamp you can install a diode on the circuit board between the DC separator and the voltage to the amplifier semiconductor that will block reverse polarity and another diode in parallel over the amplifier that conduct when you apply reverse polarity. Then if the amp stops working you simply reverse the polarity from the bias-T feed to bypass the amp and still get some signal until you repair the amp.
When you have a feed it probably are only one site you monitor and directional antennas are an option that doesn't need any amplification.
You write that the major cost factor will probably be the coax, not even the antennas. Then absolutely go RG6 that isn't much worse than LMR400 and if you add a $25 preamp and a $10 bias-T you will have all the signal you'll need for any kind of splitters and bandpass filters.
If a displexer turns out to be expensive you can have dual RG6 without it costing much and one issue with one antenna and coax will not affect the other feed.
/Ubbe