Hello,
Easiest way would be to use either
1/ Flex Link on CSCO side https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3550/software/release/12-2_25_see/configuration/guide/swflink.html
or
2/ Redundant Trunk Group (RTG) on Juniper side
But not both at the same time.
HTH
Thx
Alex