Originally Posted by ilmor
If you currently have one battery and are replacing it with the one in the picture I see no reason not too. More important is that the battery is maintained properly.
The OP said " I would like to just add a cheap similar battery in size and then when they need replaced in a few years get two new better batteries. -"
To answer your question, you need to keep the two batteries in balance, otherwise they will not be able to give you the desired performance you are looking to get by adding one.
So, yes you can add a "cheap" battery, but, you will not get the best outcome, because they are not matched, and thus performace will be diminished from what you might expect.