Hi Andre,
What is the version of VOSS you have?
From what I remember, recently there should've been added support for redundant adjacencies that are established but kept dormant until needed.
MLT would be a good approach here, you would be able to fully benefit those links.
Oh, here:
https://documentation.extremenetworks.com/release_notes/VOSS/9035336_ReleaseNoteVOSS_7.0_RN.pdf
"IS-IS parallel adjacencies between two nodes
This release supports multiple parallel adjacencies between the two nodes to provide link
redundancy. The switch selects the adjacency with the shortest path as the active adjacency.
Note:
IS-IS parallel adjacency support is enabled by default, and it cannot be disabled.
For more information, see Configuring Fabric Basics and Layer 2 Services"
If I'm not wrong, you could be satisfied with that. Personally I would go to MLT though.
Hope that helps,
Tomasz