The strategy I would probably go with in most situations has/is used on Roshar, where you give the blade to someone else but have it bonded to the person wearing the shard plate so they can summon it in an emergency.
The situations where I believe it would be better to use both together include attacking a fortified position, engaging in confined spaces (like a canyon or cave—maybe not that one, but it depends on the circumstances—there are a lot of parentheses here), or when facing another shardbearer.
If you are trying to hold a fortified position, I would utilize the plate as a tank to cover any gaps in the defenses, and I would place the blade on top of a wall (if possible) or near a chokepoint.
When attacking on a flat battlefield with roughly equal forces, separate them. If their line is wavering, send one in with both to break through. If you are outnumbered or losing, keep them apart. In a last stand where the outcome guarantees everyone’s demise, separate them.
If you are charging an army, keep the plate and blade together. A blade alone won't protect you from many threats—lots of spears for one and arrows for another. However, a plate and blade together can decimate a line of soldiers who aren't prepared.
If the opposing army has a shardbearer you should probably send in your plate and blade together to fight them, or you could try to be sneaky and lure in the opposing shardbearer by separating your blade and plate and encouraging the opposing shardbearer to attack one (preferably the plate) and then cutting off the support and surrounding the shardbearer or similar trap.
This went way longer than I expected, but I think it contains a lot of good strategic info about how to best use your blade and plate in a battle. If you have any more strategy-related questions, you know where to find me.