This report summarizes research carried out to investigate energy transfer in an activator-activator system; that is a system for which energy transfer is reversible between certain metastable levels and for which stimulated emission from each of these levels can be generated. The specific system investigated is Er-Ho:YLF. Laser operation from two different ions in the same solid state host has been demonstrated. The 750nm (5S(2) - 5I(7):Ho) and 850nm (4S(3/2) - 4I(13/2:Er) transitions were observed simultaneously and separately at 77K. Cross transfer between the S levels was also observed and the cross transfer rates measured in crystals of five different compositions. Cross transfer between the S levels is mitigated by other cross relaxation transfer schemes which quench the S level storage time of each ion. Only about 25% of the initial population of one ion is transferred to the other in the optimum concentration, 2%Ho-2%Er, at room temperature. The cross transfer rate is only weakly temperature and concentration dependent; at low temperatures cross transfer is somewhat more effective as the decay rates of each ion are longer.