Yes, that's pretty well what it means. It's a suggestion, so 'Let's', 'What about ..?' or 'Say we..' would all work as translations. As Marie points out, the expression should be 'faire des courses'.
'Si nous allions faire des courses toutes les deux demain?'
'Toutes les deux' means 'both' (feminine plural) so a possible translation of the whole sentence would be:
'Why don't we go shopping together tomorrow?'
Note that there's often more information about number and gender in a French sentence than there is in the equivalent English one. It's clear from the grammar of the sentence that this is one female speaking to another female. If you wanted to get that idea over in the translation, you'd need to say something like 'Why don't we two girls go shopping tomorrow?' or 'Let's have a girls' day out shopping tomorrow!'