Is it possible to have ASU/Income Protection Insurance with more than 1 company with total benefits exceeding £1500 per month?

I thought I read somewhere that if you tried to claim then you could only receive benefits of £1500 (or a % of your income) no matter how many policies you had.

If this is the case, how do insurance companies know what other policies you may have?

I am in the process of reviewing what cover I have and seeing what is the best level of cover for me.