I have a TP-link TL-R470T+
Not a particularly up-market bit of kit, but it seems to work OK for home use, and it cost £30, so not much lost if it doesn't suit.
If you're looking for max throughput, then this probably isn't for you, but if you just want to have 2 lines coming in with 'invisible' sharing and switching between then it may be a reasonable cheap option.
As you said, any one session must go through a single line, but I have certainly seen cases where I have one device with multiple browser sessions that are split between the two lines. (although you can stop this happening in the config).
All the normal QoS, VPN, DoS protection etc. stuff is there, and prioritising devices to lines with fail-over to the other all available. (So, I split my sons to a line each. If one wants to stream sky-go on his laptop, it doesn't [censored] the other's XBOX game.)
Probably not what you're looking for, but for me, it works OK.