If you mean in parallel, just set the ports up the same as each other and connect both ToR switches to them and also the MS225s to both.  Make sure that they have different spanning tree priorities with the existing one having the lower number.  Also make sure the Dell switches are set to MSTP.  The spanning tree protocol should close ports to stop a loop being created.  I would also link the two 390s together with a 20Gb portchannel so that that stays up and open in preference to the 1Gb and 10Gb other links.  To get the other switches to prefer the connection via the existing 390, you could make sure those are all portchannels (2x1Gb or 2x10Gb) with the connections to the secondary 390 being only single.  STP does generally prefer the faster connections.   I have done this on a smaller scale with three stacks of switches connected A-B 10Gb, A-C 10Gb and B-C 1Gb.  B-C was always STP disabled unless there was a problem with either A-B or A-C links. ... View more