Merge 2 mbitles together

[BenOnTrack]

I generate 2 mbtiles:

• output1.mbtiles
• output2.mbtiles

output1.mbtiles contains the following source layers : route_foot and route_bike (mostly LineLayer representing foot/bike ways)
output2.mbtiles contains the following source layer : route_poi (a layerAsCentroid of the above route, generated via a GeoJson input and the geojson processing with tilemaker)

When both .mbtiles are added separately to the map, everything works as a charm. Since they're both related to route I had the intent to merge them together.

First, I generate output1.mbtiles

./tilemaker --input osm_pbf --output output1.mbtiles --process route.lua --config route.json

then, i generate and merge output2.mbtiles within output1.mbtiles

./tilemaker --input osm_pbf --output output1.mbtiles -- merge --process route_poi.lua --config route_poi.json

the file output1.mbtiles gets updated, but then the entire data seems corrupted (some data disappear, some zoom are different, ....) while both mbtiles initially have completely different source layers (so I would not expect any glitch or corruption due to potential overlap)

What am I doing wrong ?

Unless that's not the intent of this "merge" function - on the RUNNING.md example provided, the objective seems to merge similar source layers based on different .pbf input - while my objective is to merge 2 mbtiles together (maybe I should use Tippecanoe?)