Say you have two ISP's and sometimes one takes ages to download a web page. Sometimes the other ISP takes ages. If you try to use them in parallel with something like pfsense multiwan, half the connections go through the one ISP and half through the other. We don't want that. Does software exist that will download the page twice in parallel, once from each ISP, and ignore the copy that arrives last?
I am not aware of such a software. It sounds more like researching why the ISP connection(s) is so spotty on your end, would be a more thorough approach. Do you know what the download speeds on your ISP connection are running? Are the speeds YOU are getting basically equivalent to the speeds they advertise? My ISP consistently exceeds their advertised speed for my home. From experience I have seen what you describe as happening while using WiFi that is causing errors. Are you wired (Ethernet Cat6) or wireless? I know this isn't actually directing an answer to your question but it is approaching a long term solution. Excuse me if you just want an answer to the software and not circuit checking. Trying to help.
If all your ISPs support MPTCP, you can fully aggregate connections to them. But that only works for TCP traffic. You could try running a TCP-mode VPN server on a VPS with gigabit uplink, and so tunnel UDP traffic. I've found mptcp-dev <mptcp-dev@listes.uclouvain.be> to be quite friendly.
Looked into link aggregation a while ago, can't remember it downloading a page twice. Are you sure it has such a mode? Using two Cat6 ethernet cables and respective interfaces and ADSL modems/routers. 14 mbps each at best. Sold as 24 mbps each.
This looks promising: "Other possible use cases are data duplication to enable guaranteed delivery and guaranteed minimum possible latency" http://vrayo.com/how-to-set-up-a-bonding-vpn-connection-in-linux/
