Given that you cannot import multiple master public keys in to the same wallet in Electrum, then your only way of doing this will be to import all the addresses individually from each wallet in to a single watch only wallet.

Rather than copying and pasting each address one by one, in each wallet you want to watch go to the console (if you don't see the console tab then click View -> Show Console) and enter the following command:


It will spit out a list of all your receiving and change addresses visible on the Addresses tab, so you can copy the whole list at once. You'll still need to manually remove the " and , symbols before attempting to import them, though.