Quote from LodeRunner:
Synthetic positions aren't any more stable. Payoffs are always exactly the same (if ever they aren't, arbitrage will instantly eliminate the inefficiency), other than the higher commissions. It's a bit hard to visualize in the case of Forex, but it should be clear if you imagined all of the currencies being priced in terms of some idealized hard, static currency.
My terminology may be off, but I assumed an actual Forex 'spread' (as opposed to merely a synthetic position) would be to long or short a single currency vs. a bunch of others. So if you think EUR is going down, but you're worried that other major currencies will follow it, you just short it vs. everything. As long as it falls vs. the average of the basket of currencies you are shorting it against, you make money. For maximum effect, you weight the basket based on observed past correlations. (Actually I think IB offers something like this.) Maybe that's the kind of 'stability' you were thinking about?
Either that, or you're just talking about exiting your exposure for one currency without affecting your bullish/bearish sentiment on the other. For example, adding EUR/JPY to a preexisting USD/EUR position, thus giving USD/JPY. This makes sense if your opinion of EUR changed (but your opinion of USD didn't.) However, I don't think it ever makes sense to place those trades simultaneously, unless your broker doesn't offer your desired currency pair.