Dynamic Hosts and Parallel Tasks with Fabric library

Is there a way to dynamically modify hosts in between multiple parallel tasks? This is what I have so far.

def init_hosts():
    env.hosts = [host1,host2,host3,host4]

@parallel
def task_1():
    if condition is False:
        env.hosts.remove(env.host)

@parallel
def task_2():
    run('uname -s')

Obviously I'm missing some env paramenters, but I only want task_2 to run on hosts that satisfy the condition in task_1. It appears task_2's host list is initialized on startup, because it's running on all hosts in the initial env.hosts list defined in init_hosts(). I also tried dynamically modifying and building roledefs, but had the same result.

Edit: Also, is there a way to setup a parallel execution queue such that multiple parallel tasks are executed in parallel rather than sequentially?

Edit: I managed to get my desired end result by having each task return output, and parsing the output to build a new host list to pass to execute():

def init_hosts():
    env.hosts = [host1,host2,host3,host4]

@parallel
def task_1():
    if condition is False:
        return False
    else:
        return True

@parallel
def task_2():
   run('uname -s')

def run_tests():
   results = execute(task_1)
   successful_hosts = [k for k in results.iterkeys() if results[k]]
   execute(test_2, hosts=successful_hosts)

This works, but it's gross for many reasons. Is there a better way?