[More musings. I started writing this post arguing that they are joint inputs. Now I'm not so sure. So I changed the title into a question.]
We need both land and labour to produce wheat, but land and labour are not "joint inputs", because we can combine them in variable proportions. The marginal product of land is the extra wheat per extra land, holding labour constant; the marginal product of labour is the extra wheat per extra worker, holding land constant. Both are well-defined under variable proportions. If we are combining land and labour optimally (to minimise costs of producing a given amount of wheat), the envelope theorem (IIRC) tells us that the marginal cost of an extra ton of wheat will be the same whether we produce that extra ton by: adding extra land to labour; adding extra labour to land; or adding both extra land and extra labour.
But suppose we could only produce wheat by combining labour and land in fixed proportions: one acre of land plus one (full-time) worker produce one ton of wheat. Two workers and one acre produce one ton; two acres and one worker produce one ton. The marginal product of land is either: zero (if there is more land than labour); one (if there is less land than labour); or undefined (if there is equal quantities of land and labour). The same for the marginal product of labour. Land and labour would then be "joint inputs". We could only talk about the marginal product of the composite input land+labour (one acre of land plus one full-time worker have a combined marginal product of one ton of wheat); and the marginal cost of wheat would be the cost of the extra land+labour needed to produce an extra ton of wheat.