Although the combination is a little weird and may limit its use, seem a bit powerful.
Looking at the poison counter and cumulative damage dealt:
Turn | Poison Counter | Cumulative Damage |
1 | 1 | 1 |
2 | 3 | 4 |
3 | 6 | 10 |
By the end of the third turn, the enemy creature would have been dealt 10 damage total, with almost no chance to survive the next turn.
Additionally, the upped 'death adder' would be able to be played as an offensive creature (player damage), in addition to cc.
I would suggest raising the cost of the unupped card to 5
and decrease the upped version's attack to 2.