The goof-proof way to make overlapped (tapeless) seams is to use a removable joggle-gasket so that the overlap flaps are actually molded in. This is a level of complication that most modelers don't want to deal with.

Short of making these special mold pieces, you can still do the overlap method but it takes a lot of practice and details of the molds can make a large difference in how tough the job is. For example, if you have straight mold locating pins (like 3/16" music wire), then it can be nearly impossible. It's much easier if you have tapered locating pins (like hemispheres as an example), so you can bring the two sides together at an angle. The flaps are much less likely to catch and hang up on things this way. A picture would be worth a thousand words here, so I'll steal a diagram from the R&G user guide (http://www.r-g.de/):