It's not a function.. it's an event.
Of course it won't work.. it gets triggered for each file separately.
Use something like this:
local filesDownloaded = {}
function onDownloadFinish ( file, success )
if ( success ) then
filesDownloaded[file] = true
if ( filesDownloaded["solider.txd"] and filesDownloaded["solider.dff"] ) then
engineImportTXD(engineLoadTXD("solider.txd"), 16)
engineReplaceModel(engineLoadDFF("solider.dff", 0), 16)
outputChatBox("soliderload")
end
end
end
end
addEventHandler ( "onClientFileDownloadComplete", resourceRoot, onDownloadFinish )