To combine all upload bandwidth across two DSLs for a single session you need to use SpeedFusion Bandwidth Bonding to another remote Peplink appliance that has enough bandwidth to receive your upload and forward on to the internet.
From a device perspective you could consider a Balance 210, or a Balance One with the additional SpeedFusion License. Then you could host a FusionHub Virtual Appliance in the cloud (AWS, Azure etc) as the remote device.
However depending on what your uploading you could achieve the same result with software on your devices. For example amazon S3 supports multipart upload now, so if you are uploading files to share with others you could use that.
Also I would recommend https://cyberduck.io/ it supports multi-part / multi session uploads for loads of different protocols / services. .
If its just general uploading of files to multiple websites then you will need to look at SpeedFusion Bonding.