There already is a key program that achieves what your essentially trying to do, Economics and Finance. By doing Economics and finance you essentially end up doing all the 2nd yr units you would do in Applied Economics or Applied Finance. However some units that are specific to Applied economics or Applied finance in the 3rd yr would not be completed if you done the Economics and Finance key program.
Applied Economics or Applied Finance has 16 units that you must complete to graduate, Economics and Finance has 18 units. Keep in mind all of the units you do in the first year are identical between the key programs mentioned so your not missing out on alot of content by doing a "double key program".
Im pretty sure you can only select one key program on MYUWS, they wont let you at all select two key programs even if you do qualify for it. If you dont want to do Economics and Finance you can also do a major in one of them. You only need 24 units to graduate so there is nothing stopping you from having more then 24 units(correct me if im wrong), this can be quite difficult to achieve since you would need to do summer school or take on more then a fulltime load of units. Hope that helps.