You made a loop (well, in reality...you made two loops...one loop within each backplane stack!); it's crystal clear.
Consider that each Backplane Stack represents a single logical entity so you shouldn't create a loop between ports of the same Stack (e.g. between 750:1/B1 and 751:2/B1 for the top stack and/or 754:1/B2 and 755:2/B1 for the bottom stack).
To create an aggregated links "uplink" between both stacks you could instead create a LACP (IEEE 802.3ad) aggregated trunk "TrkX" (let me say Trk1 on both ends) respectively made of 1/B1+1/B2+2/B1+2/B2 interfaces (Grandtotal of 40 Gbps) of top stack and another TrkY made of 1/B1+1/B2+2/B1+2/B2 interfaces (Grandtotal of 40 Gbps) of bottom stack...and interconnect them 1/B1 to 1/B1, 1/B2 to 1/B2 and so on. Simple.
The LACP configuration you linked is OK.