"play them freely"
If there are not 2 thirty-second notes for each sixteenth note, then what would the rhythm be?
All Eddy is saying is that the "margin suggestion" to play the trill as demisemiquavers should be taken with a pinch of salt.
If you are saying that the right hand has to play this trill while the left hand has ordinary semiquavers, then I can see your problem. You may not want to play the trill as semiquavers also, but do they really need to be in a simple ratio? If so, I suppose you could try 3 trill notes for every 2 semiquavers, giving a speed ratio of 1.5 instead of 2, but ideally your brain should be able to instruct your hand to play the trill "as fast as possible" without
any well-defined speed ratio.
No general advice on this issue will be applicable universally, there will always be exceptions. Would you like re-ask your question giving a reference to a specific place in a specific piece?