Consider the sentences:
If you agree with that, could you suggest us some references?
If you agree with that could you suggest us some references?
Which one is written correctly? Notice the use of "," in (1).
Not only is the comma fine, it is actually useful. Without it, the reader might parse the that not as a demonstrative pronoun, but as an adjectival that, expecting a noun to follow and getting caught off guard by a could. You'd be putting an unnecessary hurdle in his way.