Joining two wires

What is the safest way to join two 600 v wires togeather?
These wires are under the chassis and come from the power transformer.I am worried that once I solder them togeather that shrink tubing won't be enough insulation,will this be ok or is there another method?
Solder them togheter but use two to three pieces of heatshrinking over eachother - it will provide good insulation...

600v but low current?

Depending upon wire gauge, there's a thing called a but connector. they are insulated open ended barrels to which the ends of the wires being spliced are inserted. the barel is them crimped... you're done. Applying heat shrink or plain electricians tape will suffice then to cover and protect the splice.

Saves soldering. high voltage, high current applications are done with compression... crimped... not soldered. soldering just adds more to the path... and resistance. securing the connection is the key. solder is appropriate where crimping is not.