I print double sided banners, obviously some designs do not lend themselves to be double sided eg: a background that has a patten in that bleeds of the edge.
But the technique i use on my mimaki jv3 for white or solid background colour is to print first side with crop/registration marks in each corner, when dry turn over locate the crop/registration marks with a pin from the underside and mark the unprinted side with a pencil, do this for all 4 marks.
Then feed the banner in and use the first pair of marks at one end of your banner as your datum point where to start printing.
You have to know where the exact point your printer will start printing in the x and y axis, and you have to make sure you start to print at the right end for the reverse of the banner otherwise you will end up with the print upside down to the other.
Also make sure you feed in straight, i can achieve a 1.5mm accuracy with this technique.
For me this easier to do than to explain the process, so apologies if this does not make much sense.