I am not familiar with the term double up welding. Please clarify.
And to get a complete metallurgical answer you may wish to provide us with the material you are talking about as well.

Here is some excerpted text that may be of interest from: Magnitech:::
http://www.magnatech-lp.com/articles/orbitalgmawfcaw.html

"Orbital or Double-Up Welding?

While the term orbital has become closely associated with mechanized GTAW welding, it is not totally appropriate when applied to mechanized FCAW welding. With the pipe in the horizontal (5G position), welding is done using a double up technique - 180 clockwise followed by 180 counterclockwise. An orbital technique is used only with the pipe vertical (2G position).

A double-down welding technique can be used with GMAW. This technique is generally only used for cross-country or marine pipeline laying operations. Joints-per-day productivity of these operations mandate multiple weld stations with two or more weld Heads per station used on a joint simultaneously. Each station is usually dedicated to one pass - for example, a Root Pass station followed by Hot Pass stations, two Fill Pass stations, and the final Cap Pass station. As each pass is completed, all the stations move forward to begin the same pass on the next joint in sequence. Because all stations must move-up in lockstep, the weld cycle time is dependant on the speed at the initial Root Pass station. While much higher rotational travel speeds can be used, less metal is deposited per pass. Great care must be taken by the operator to avoid sidewall fusion defects."