So, offer something else.
The UN observers:
un.org/womenwatch/osagi/wps/publication/Chapter7.htm
The United Nations is the only organisation that would be viewed as objective enough by both sides - Ukraine and RuSSia, I think.
Although, in all honesty, in my opinion having only UN observers wouldn't be probably enough (in case of RuSSian occupation - especially). The only way to ensure for such referenda to be really free, fair, with everyone elligible having the chance to vote and not being afraid to vote the way they want and both sides being able to freely agitate for their cause, would be to set up a special UN mission in those regions with the UN temporary supervision of those territories (with "blue helmets" on the ground, I imagine) and the UN being in charge of the referendum, like in this case:
minurso.unmissions.org/background
Those would be
the ideal circumstances and noone would contest such results then (including me).
The only thing that really matters is if the voters in both refererenda were in any way threatened to vote yes or else.
Of course that's not the only thing that really matters. All kinds of other things matter if you want to have a free and fair referendum with credible results. Including whether people were afraid to vote "no". And judging by all we know and based on the reports available - they had all the reasons to be afraid.
Prove it or shut the fu*ck up.
I don't have to prove that those sham referendums were conducted on occupied territories by the invading forces - we all know this. Such referendum shouldn't take place in such circumstances in the first place. If Ukrainians or any other forces invaded RuSSia and conducted "referendums" in the same circumstances, their "official results" would be contested too (including by Velund and you = so, you're total hypocrites) and rightly so.
The way those "referendums" were conducted broke even RuSSian law, which makes those referendums illegal even under the RuSSian law.
RuSSia doesn't even care about it's own laws. This means RuSSia doesn't have any integrity at all and is simply not trustworthy. Those "referendums" were a joke, a farse.