Would it be possible to use two UPS in series?
Would it increase/double the protection time (battery capacity) or it would just don't work?


While, depending on the exact models, it should work, this is almost never going to be an efficient system.


Most UPS units (ok, all except for ones made for powering 'active power-factor correction' equipment) produce square-wave output, which isn't what the 2nd UPS was designed to use as input, so may not be converted/used as effectively by 2nd UPS ("Modified Sine-Wave" is marketing-speak for square-wave).  
UPS untis incur losses while monitoring line conditions, charging the battery(s), and generating AC output from the batreries. Daisy-chaining them like this is akin to converting all of their power wasting to resistors, then placing them in series before your load. While the built-in boost converters raise the voltage back up to what your load needs, the power losses are compounded.



  Same about putting them in parallel? AFAIK, it would not work because it would require the two phases to have exact same frequency and be in phase (which by default is not the case).


If you were to purchase a pair of UPS units that were purpose-made for paralleling onto the same circuit (with built-in phase-alignment/synchronization capabilities), this could be a perfectly valid option.

Ultimately, the generally-preferred method is going to be to simply parallel batteries in one UPS unit that has sufficient drive (watts/VA) to power your equipment. This gives you the longer run-time of 2 parallel batteries, without the added power losses/waste of using 2 UPS units.

NOTE: For most efficient/effective use, you may want to look into putting shunt-resistors (usually around 0.002-0.01ohm) in series with each of the parallel batteries' positive terminal in order ensure that neither battery takes "more than its share" of the charging/discharging stresses.