For an instantaneous square wave, these method don t work. If your tape deck has only a single set of play and record connections, you can not loop the signal back as these unit are just like single cassette unit. The only real solution is to have separate cassette units. Look, cassette players and recorders are real cheap in cost. You can get a personal walkman type tape player for a play deck and use the good deck for recording.
All mixer can handle a line level head set signal into a line level input channel.
However, most walkman type units that can record too can not accept a line level signal input. If your church can afford replacing and repairing speaker on a regular basis, ignore this message. If your church is looking to cut expenses, for the cost of less than most speaker repairs, your church can afford that second tape cassette unit. There are a few Double Dual tape decks that have two sets of record and play connections, then you can use these units as two separate tape deck. However, some of the units have a switch that links the two decks.