I'm currently programming a version of the go/no-go task in which participants need to make a double-response in one condition recording 2 responses.
As such the trial has been programmed this way: 

<trial practicedoubleleft>
/ ontrialbegin = [values.cue_side=1]
/ pretrialpause = 0
/ posttrialpause = 0
/ validresponse = (46, 50, 57, noresponse)
/ correctresponse = (46, 57)
/ stimulustimes = [0=fix_green; 1000=practiceHS]
/ beginresponsetime = 1000
/ timeout = 2500
/ ontrialend = [values.double_time = 2500 - trial.practicedoubleleft.latency]
/ branch = [trial.p_double_l]
</trial>

<trial p_double_l>
/ ontrialbegin = [trial.p_double_l.insertstimulustime(clearscreen, values.double_time)]
/ validresponse = (57)
/ iscorrectresponse = [trial.p_double_l.response=="57" && trial.practicedoubleleft.response=="46"]
/ stimulustimes = [0=fix_green, practiceHS]
/ timeout = values.double_time
/ pretrialpause = 0
/ posttrialpause = 0
</trial>

The issue is that in the second 'trial', the practice HS stim changes whereas I need the stimuli to remain the same in both of these trials.
I have tried /erase = false in the picture element but this hasn't worked.

Thanks in advance 

What you need to do is use <values> and a <list> to select the HS stim item, and then you need to only do that at the start of the 1st trial, not the 2nd one. In short:

<values>
/ itemnumber = 1
</values>

<block exampleblock>
/ trials = [1-4 = first]
</block>

<trial first>
/ ontrialbegin = [
values.itemnumber = list.itemnumbers.nextindex;
]
/ stimulustimes = [0=fixation; 1000=examplestimulus]
/ timeout = 2500
/ validresponse = (46, 50, 57, noresponse)
/ correctresponse = (46, 57)
/ branch = [
trial.second
]
</trial>

<trial second>
/ stimulustimes = [0=fixation; 1000=examplestimulus]
/ timeout = 2500
/ validresponse = (57)
</trial>

<text fixation>
/ items = ("+")
</text>

<text examplestimulus>
/ items = exampleitems
/ select = values.itemnumber
</text>

<item exampleitems>
/ 1 = "Example A"
/ 1 = "Example B"
/ 1 = "Example C"
/ 1 = "Example D"
</item>

// 4 items in this example
<list itemnumbers>
/ poolsize = 4
</list>