As Rivets said Robert. Cut out the required section.
I would mark where i wished to do this, then cut with tube/pipe cutters. Don't force the cut action because you will 'fold' in the tube more than necessary. You could part cut and finish with a hacksaw too, if you wished.
Anyway, clean the tube ends with a file and insert lengths of alloy or use the cut out tube sections. If you use the cut out section, cut them lengthwise with a grinding disc ( be careful here) What you will end up with is over size tension pins:wink:
Insert/drive these in equidistant and rivet in place. If the handle tubes are too light in section, scrounge up some heavier wall material and follow on as described.
Or take it to your friendly welder and get him to TIG/Mig the join. Any engineer will know how to do this successfully.
By the way, NICE mower.
Edit:
Good grief!! I just realised how old this thread is lol
I assume you have the handle sorted by now Robert?? :biggrin: