I've received offers from two UK universities, first conditional and second unconditional. There's always a possibility that I won't meet conditions so I'd like to keep the second university as a back-up. Is it possible to do it like this, that I accept both offers and after I meet conditions, I will let the second university know, that I decided not to start studying?
In principle, you could do that, but it's not advisable, for the simple reason that if either school finds out you've accepted another offer, then both schools could end up rescinding their offers, and would be within their rights to do so. (Programs want to know how many spots they have filled, so that they can accept more people if needed to meet their enrollment targets.)
Basically, you need to commit to one school only. If the offer is conditional, then you should weigh the probability of meeting those conditions before accepting the offer.
Yes, for UK undergraduate admissions, you can accept two offers at once - but no more!
One will be your first choice; if you don't meet the conditions, then you have a shot at the second one. So in your case, you would put the conditional offer first, and the unconditional one as your "insurance". (The other way round doesn't make sense, as you can't possibly miss the conditions on the unconditional offer!) You could also just accept one offer.
Make your choices carefully, because the subsequent process is automatic - if you meet the conditions then that's where you're going, unless you manage to convince the university to release you. That is technically possible but should under no circumstances be relied on as a strategy.
I have assumed you are applying to become an undergraduate, as the language of conditional and unconditional offers is particularly associated with this system, and is very standard. If you are applying for a postgraduate position, or for a job, then please disregard this answer.