My first thought is what size connection does the backup appliance have to the switch? Gigabit? A pair of 10Gbe?
If you ignore redundancy, a 4 port Etherchannel from the MS225 to a single MS420 will give the most throughput and is particularly easy.
You could stack the two MS420's and then split the 4 x 10Gbe connections across them in an EtherChannel. The only downside is when doing firmware updates both switches will get rebooted at the same time. If this is not an issue, then I'd do this.