It doesn't matter if you have them grounded to the same spot or not. All that matters is that it is a good ground. Strut or seat belt bolts are generally the best. You don't need to worry about anything else. Unless your doing a big system, don't waste the money or time running another wire to the front battery.
Make good grounds and be done with it. If it won't fit over the seat belt bolt or strut bolt, put a bigger terminal on it