You could specify the transformations for each dataset.
This code should work:
mnist = datasets.MNIST(
    root='/home/ptrblck/python/data',
    download=False,
    transform=transforms.Compose([
        transforms.ToTensor()
    ])
)

cifar = datasets.CIFAR10(
    root='/home/ptrblck/python/data',
    download=False,
    transform=transforms.Compose([
        transforms.Grayscale(),
        transforms.Resize(28),
        transforms.ToTensor()
    ])
)

dataset = ConcatDataset((mnist, cifar))

As you can see transforms.Resize and transforms.Grayscale was only applied for the CIFAR10 dataset.