The Code does not forbid this - else how could you butt-weld two spools of seam-welded pipe together? Of course, with many seam-welded spools, you can alternate the seams at 10:00 and 2:00, but you can't always do this when you get two shop-fabricated assemblies to the field. Also note that welded on reinforcing pads at fabricated branch connections frequently overlap both longitudinal and girth welds.
You raise a good point, though, in that such details should be avoided when possible. You can also manually enter a weld joint factor of .85 (or whatever your judgment believes to be prudent) for the region where a cross weld exists. Most of the time, such details aren't located at peak stress points, since those are typically at elbows and tees for well-supported piping systems.
Pressure vessel shops routinely go to some lengths to avoid cross welds, but they have all the pieces in-house and under their control. They also will go to great lengths to avoid putting nozzle openings on seam welds between two plates. One reason for this is that pressure vessel walls are typically stressed to a much higher percentage of yield than all but a few locations in any given piping system will ever see.
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CraigB
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