It is perfectly acceptable to give both, but only if really necessary - for example the child has an extremely high temperature that is not responding to calpol alone. Ibuprofen is excellent for lowering a temperature, but isn't as gentle on the tummy as calpol. You should never use ibuprofen is the child has a tummy ache( except under medical advice of course!)
However what you should do , is give the medecines alternately. ie. give a dose of calpol and if more medicine is required before the next dose of calpol is due, you can give ibuprofen. etc. You must be careful not to exceed the dose of either and if the temerature or whatever is not responding, then you must seek medical advice.
Incdentally, you cannot develop immunity to pain killers, but I would also recommend using them only when absolutely necessary, there are others ways to lower a temperature, sponging with a damp cloth etc.