I am curious of this also for my old underpowered cellphone. here I am looking at this page with chromebook / chrome browser, both ublock origin and ghostery running. both are geared for maximum defense/blocking of everything possible.
ub has 4 items blocked and ghostery has 5 blocked.
I just tested and reloaded several times turning one off, then the other, both off, then turned them on in opposite order. Was thinking that one was blocking some elements so the other never saw them to block them but the counts were the same with or without the other defender running.
9 items on this page being blocked by using them simultaneously. so for what is worth, I satisfied my curiosity on needing to use them both. if my little dinosaur can handle them both without heating up or crashing the rooted eclair, i will run them both.