Do not do this! If you wire two cells at different states of charge together in parallel, there is nothing protecting one cell from dumping massive current into the other to equalize the voltages. This can be very unsafe with lithium cells, leading to one cell bombarding the other with tens or even hundreds of amps with catastrophic results.
Lead acid batteries can provide huge bursts of current and have a maximum rated current at which it is safe to charge them. Beyond this, you are boiling off electrolyte and risk damaging the plates.
Connecting a battery at 12.6V in parallel with one that is at 12.0V, for example, is not a good idea, especially if they have vastly different capacities - like a home UPS battery and a tiny starter battery for a motorcycle. You'll likely damage the starter battery, because it'll take in as much current as it can, limited only by its own state of charge, the wiring, and what the other battery can deliver, which is almost certainly higher than the C-rating of the smaller starter battery.
You can charge your bike battery with a UPS in an emergency, but please do not connect batteries with different voltages together in parallel. The correct way is to use a current-limited, constant voltage power supply or charger. Hell, you can even use any DC power supply rated from 13.8V to 14.4V to charge the battery at 1A or 2A if you monitor the battery voltage and disconnect it when it reaches close to 14V.
If you must use a UPS, don't let the battery out of your sight. Keep an eye on it with a voltmeter and make sure it doesn't exceed 14.8V.