AIUI▸ they have to be careful to not be accused of co-ordinated action to cause the most disruption, so choosing the same day removes that risk I guess.
AFAIK▸ , nothing prohibits trade unions from coordinating strike action.
What IS prohibited is sympathetic strike action whereby trades unions who are not in dispute, refuse to do certain work in order to help support other disputes.
The classic example was rail workers refusing to handle coal trains, NOT because said rail workers were in dispute, but to support the coal miners who were in dispute.
Or Royal mail workers refusing to handle post for Grunwick, a film developing firm involved in a dispute.
Such cases used to be common but are now prohibited.