IIRC using a splitter "in reverse" to combine two identical aerials fed exactly in phase still won't give you a purely 3db gain due to the power losses inherent in the splitter, especially resistive splitters as opposed to inductive splitters. Having used Fringe UHF-UHF diplexers in the past their insertion loss for the passband was usually given at 1db.
In the days prior to easily receiving BBC TV broadcasts via satellite in non-cabled areas, some of the terrestrial TV aerial setups in many parts of the Republic of Ireland to receive terrestrial stations from NI or Wales were a near art form. I remember when going to the likes of Dublin the size, height & even combos of UHF aerials installed in places grew bigger the further south you went (and west to a certain extent). It went from having a standard 18 element Group A in Cavan town (to pick up Brougher Mountain) to seeing some monster setups in north & west Co. Dublin often involving four very high gain UHF aerials phased together and even spotting a parabolic UHF reflector on the roof of a pub (the sort of aerial Channel Master in the USA used to have as its top "fringe" UHF aerial) - in both cases a decent masthead amp was also being put to use. In one case in Co. Meath I recall a house close to the roadside whose northern-facing aerial was on top of a tower that was at least 15 metres tall - the next time I went past it there were GSM aerials placed below it!
