nope. you can use 1 until it caps and move to the other to have a limit of 2gb. but you cannot sync two separate devices to tether a single stream of data.
There is 3 ways but it's tedious First is to either have a certain program which is bundled with a certain hardware and I doubt you would spend that much for it as it is expensive and it only slightly increase your speed through application management. Second you would need to write a program yourself cheapest but you would need programming knowledge on splicing the network upstream and encoding it back into 1 piece in the downstream. This would double your speed in your case. Theoretically but it depends on the stability of each of your connections. Third you would have a certain hardware which I think is called network splicer or something... which could help do the decoding and recoding in each connection to get specific data and work itself before reaching your device and double your speed in your case and it will be able to manage the connection on the weight of each upstream and downstream to ensure a more smooth and stable connection... The downside of the hardware is that it's an Enterprise hardware so it's expensive and you would need a rj45 connection which means you would need to put each your sim cards into 2 different sim enabled router... Of course there is a dual SIM router but that only uses the other sim as a backup connection... So no dice there. The easy answer to your question is there is a way but if you can't program it yourself or you don't have enough money then forget it.
There are some options, but they are generally expensive and are used for mobile businesses or traveling technicians. If your phones have SIM cards, you can get a router that will work with two different cellular modems that you connect to it, or if the router has internal cellular modems, with two sim cards that you insert into the router. Most of the routers that accept two SIM cards or two cell modems are made for redundancy in case one modem loses signal. They're not really made with link aggregation in mind. As far as I know, there's no solution to do what you want to do with two cell phones. As for aggregating your two cell phones to double your speed online, you're probably going to be unhappy even if you find a solution. Most stuff online assumes you only have a single IP address. If you try to load a YouTube video, there's no easy way for YouTube to know that you're connecting with two different IP addresses, so you'll likely have the same load times that you had before. For some things, like torrenting, you'd see a definite speed increase by aggregating your two phones.
Have you tried something like Speedify? It can combine multiple internets, though not sure if it can do multiple usb but it can do multiple wifi and wifi+usb.
hmm.. interesting. Haven't seen this service before. But seems error-prone and potentially unstable... Did you use it?