Ytdyklly

There is the application Speedify which tries to do that, but there is no Linux application at the moment so I started to think if it can be done yourself. I have NordVPN account.
The service connects to its VPN server.
I think it creates this way a second interface or something to enable the simultaneous access in two sources for internet.
Technical support answer to my question about the application VPN connection and its release in Linux

We use a VPN setup to make channel bonding possible. Without a server
component, you can only load balance between multiple Internet
connections, which means you can assign sockets to different Internet
connections. Speedify does channel bonding, which allows you to split
even single socket transfers, like video streaming, across multiple
Internet connections.

We currently do not have a Linux client available, but we might
consider offering on in the future.

Speedify is its own VPN service and is not compatible with other VPN
clients or accounts. You cannot use NordVPN to connect to Speedify.

NordVPN answer because I would like to use my own VPN to with/without Speedify for the feature

Unfortunately we are not aware if it will be working, since if Speedify is only pinging/sending packets to other Wi-fi/mobile data servers then it will work. However if it is providing some kind of encryption, then it might cause issues if it is used with our app.

Also, this app might be switching between networks from time to time. This will require you to reconnect to a VPN server every time you switch a network.

Situation: if you use Speedify, you cannot use NordVPN. This is a big limitation so I want to know what it requires to have a simultaneous connection to two internet sources.

Proposal from NordVPN

It would not be possible as from the answer provided by Speedify it looks like in order to use both Wi-Fi and LTE connection you would require to set up a VPN-like service and thus you would no longer be able to connect to actual VPN.

So I think the interface of VPN must be changed to support both connections.
I think this is what Speedify has done.

OS: Debian 8.5

Our prayers have been answered and we didn't even know it. vtrunkd is an open source SD WAN daemon for linux (network link bonding/trunking/aggregation and multichannel VPN daemon).

This would be like using Speedify but you would be running the bonding VPN server. I haven't tried it yet but it's been around for 3 years now. I'd imagine that with some determination, one could get it to work.