it's THAT easy to make a grammatically incorrect sentence in Polish, especially with numbers.
Before you carry on in this vein, please be advised that BOTH these forms are acceptable in Polish - "dwoma" + Fem is archaic, but not wrong.
Also, Polish is not the only Slavonic language to use different forms in this case - Slovak springs to mind (check the Wiki entry on dual forms for more info).
Additionally, Czech has both
dvema lahvema (informal) and
dvemi lahvemi (formal), and that's just an example, as the formal / informal divide goes right through the language, on the level of vocabulary, pronunciation, and grammatical forms - how easy do you think that is for a learner?
jsou spatne vychovany
jsou spatne vychovane
To make things worse, the choice of formal vs informal is not always clear, as e.g. a highly educated person might use informal words or forms in their formal speech to lighten the mood, or someone might, while speaking in full informal mode, use a formal phrase or form to create a comic effect, or to inform the listener that they are being serious at that precise moment, etc. etc.
Plus other fun extras like the imperfective verb forms:
Kdyz jsem tam sel, zastekal na mne pes (perfective)
Kdyz jsem tam chodil, stekal na mne pes (in the past, each time)
Kdyz jsem tam chodival, stekaval na mne pes (repeatedly, and some time ago)
Kdyz jsem tam chodivaval, stekavaval na mne pes (a long time ago, several times)
which is the equivalent of roughly "When I went there, a dog used to bark at me".
I agree that Polish is one of the more complicated languages. But I insist it is one of many, and definitely not the most difficult one. Please read up on other Slavonic languages. And that's just for starters.