Is it permissible to pray fragmented congregational prayers for both Dhuhr and Asr in a prayer room at the workplace instead of one single congregation, knowing that forming one single congregation was difficult and work can stop at prayer time?

Congregational prayer is obligatory. If it is not possible to establish it due to work necessity, then there is no harm in performing prayers in groups during the time, while being keen on having a regular imam lead the prayer. Prayers should be performed at their legally prescribed times. If that is not possible, then the Muslim should fear Allah to the best of his ability, for "Allah does not burden a soul beyond its capacity."