Some codes allow for this practice. The condition is that the weld passes and all three welders are qualified or the weld fails and all three welders fail.
Now comes the fun part, the first welder is qualified with and without backing, the other welders are qualified with backing only.
Al
Couple things. First, this practice can be done according to ASME Section IX, except for GMAW short circuit. It cannot be done per API 1104. But keep in mind, API 1104 is a standard not a code. The owner has the authority to waive. However, the issue will be, for an API 1104 regime, answering to the DOT. If API 1104 is used the DOT expects full compliance.