ALLOCATE commands can be given in sequence without waiting for the first allocate to finish. However, the same Allocate command should not be done twice, unless confident that the allocation has failed and that there is no entry in the transaction databases at either of the locations affected by the allocation.
…and so on for each allocation.
If a de-allocation is in progress (see the DEALLOCATION command), then the allocation will stall until the de-allocation is complete before commencing.