Bash: command substitution with multiple unset/export commands

I am trying to unset some variables and export others using command substitution. This works:

$(echo "
    export TEST_A=1
    export TEST_B=2
")

And this too:

$(echo "
    unset TEST_A
    unset TEST_B
")

However, this fails with Bash 4.3.30(1)-release on both OSX and Debian Jessie:

$(echo "
    unset TEST_A
    export TEST_B=1
")
# bash: unset: `TEST_B=1': not a valid identifier

Is this an expected behavior, and how can one unset and export multiple variables using command substitution?

To provide some context: here is the related issue in boot2docker.