Yoy want to "stack" device so ...
1. You can connect both them using any of SPF+ ports eg. SFP+1 on routers R1 & R2 with FO devices or DAC cable.
2. Make these interfaces parts of LAN bridges on both R1 and R2 to have one L2 domain.
3. If the SFP+ <-> SFP+ connection is too slow then you can use more of SFP+ ports on both routers and bond them to make one faster interface and then make this bond part of bridges instead of "slow" SFP+ ports.
4. For (3) you need as many FO cables or DAC cables as many ports you connect.
5. Of course you can use any of SFP+ ports mix to make a bond but always you use 1-to-1 "patchcords"
6. You can use up to 8 SFP+ of two QSFP+ ports but how to connect them if you have only quad QSFP+ cables ended with QSFP+ connectors or octopus-like QSFP+ to 4xSFP+ ones.
7. How do you want use these quadrupleconnector cables to work with single QSFP+ port?
So ...
using one QSFP+ - QSFP+ cable lets you configure one 4xSFP+(=QSFP+) link with one cable from R1 to R2.
You can use QSFP+ -> 4xSFP+ "octopus" cable and bond Q1-n ports in one router and up to 4 "simple" SFP+ ports on another but does it have sense?
If you think of redundancy then use two QSFP+ cables and make bond of 8 SFP+ subports as one bond having speed up to 80Gb and protection of losing one of QSFP+ ports. However both QSFP+ are internally connected with one line to "fabric" (
https://i.mt.lv/cdn/product_files/CRS326_180248.png)so real protection should use QSFP+ port + 4xSFP+ ports and two octopus like cables so at the end they grab 5 ports to make a protected bond.