|princess martha of sweden and king olav v of norway|
they could converse well enough to get married
swedish and norwegian :: they are so similar, it's easy to see why some people don't even consider them separate languages. the norwegian accent is much easier to grasp, which helps. you could also throw danish into this group, of course, but the danish accent is a lot different than the other two, perhaps influenced by its proximity to other mainland european countries like the netherlands.
spanish and italian :: italian is trickier than you might think, but spanish is very easy to grasp, which in turn makes italian easier to tackle. that said, the vocabulary of italian is probably easiest to link to words in english. french, while it's clearly within the same language group, has a surprisingly strong german influence that makes it quite different that the others.
russian and polish :: combining any slavic languages is fairly easy, but i think these two are the 'extremes' of the group. other slavic languages tend to occupy space that falls between them, and there are still enough similarities that you'll find it easier to master the vocabulary of both. russian has a lot of borrowed english terms that more of it sounds familiar. polish pronunciation is a great guide, which can make it easier to overcome any trepidation over having to learn another script.
arabic and hebrew :: this is a bit tricky, because "arabic" is a continuum, but they all have some similarity to their linguistic cousin hebrew. plus, it gives you a chance to learn two new scripts and you're making a quiet political statement, if you want to view it that way.
there are other pairs that are supposedly easier to master together because they are quite similar to one another, although i can't swear to it, because i haven't tried them together:
lithuanian and latvian :: the only two remaining baltic languages. they aren't mutually intelligible, but from what i understand they're closer than spanish and italian are to each other [but not as close as swedish and norwegian].
dutch and afrikaans :: the latter is an offshoot of the former, adopting its structure and much of its vocabulary. for an english speaker, the non-dutch words are largely english, and the afrikaans accent is easier to master.
spanish and catalan :: some would argue that they're not separate languages, but don't say that in catalonia.
spanish and portuguese :: to my ear, the south american varieties of both are much closer than the european ones.
finnish and estonian :: the estonian language was decimated by years of occupation and repression, so when it was resurrected, it turned to its geographical and linguistic neighbour to fill in some of the gaps.
and finally, here's a few that i think you definitely have to learn by themselves.
hungarian :: fiercely and proudly different, it'll require your entire focus, if only because the vocabulary is vastly different than anything around it.
turkish :: you'll hear its influence in a number of different languages, but its grammar is quite unique. it's related neither to arabic nor to the indo-european family.
swahili :: although it's a member of the bantu language group, it has a lot of borrowings from other cultures, which will make it an awkward fit with just about anything.
romanian :: i don't care what anyone says, it's not a romance language like the others. there are significant borrowings from slavic languages and from turkish. it's also unpredictable: only english has so many exceptions to its rules in my experience.
so... who wants to try a 'double whammy' with me? or who wants to dedicate themselves to one exquisite and unique language?