How does it work ?

To be able to use both ISP connection you must have filled in TCP/IP on every computer two default gateway. Then computer
communicate randomly from both but if you download file this file allways you use one gateway. The better is to use router (or firewall)
which is able to use two connections and load ballance communication between them.

If I use a router , then I will be able to use both connections at the same time ?
is it possible that 1 packet comes from connection1 and another comes from connection2 ?

thanks

The router (must have loadballance capability) is able to use second connection if first is at full capacity.

Just to be clear:

- a device which performs router functions (be it a router box or a computer) may be able to join internet sources if it's set up to perform that purpose.  If it serves a LAN as well then nothing on the LAN need be configured to deal with this - in general anyway.

- there are protocols and devices which deal with packets arriving out of order.  So, that should not be a concern on the LAN.  But, whether the router function you have deals with this or not would be a question.  There may be Packet Order Correction at the near end or Same Order Delivery and there may be packet retransmission.  Check the functions of the Transport Layer.

From Wikipedia:
Even though every packet has the same destination address, they can be routed on different paths through the network. When the client program on the destination computer receives them, the TCP layer (Transport Layer) reassembles the individual segments and ensures they are correctly ordered and error free as it streams them to an application.

.... this implies that a multi-homed box (i.e. multiple ports for internet service for example), must have a TCP/IP stack that deals with the composite results from the ports.  I'm not sure how Microsoft does this but I'd imagine it's buried in Bridging...