Short answer. Yes, half and half — provided they are of the same degree. Article 987 gives one-half to the paternal and the other half to the maternal ascendants where they are of different lines but of equal degree, and divides each half per capita among the ascendants in that line.
What the law says
In default of the father and mother, the ascendants nearest in degree shall inherit. Should there be more than one of equal degree belonging to the same line they shall divide the inheritance per capita; should they be of different lines but of equal degree, one-half shall go to the paternal and the other half to the maternal ascendants. In each line the division shall be made per capita.
Civil Code, Article 987 — Nearer Ascendants Exclude the Rest. Read the full provision →
Degree first, then line
The article works in two steps and the order matters. First, in default of the father and mother, the ascendants nearest in degree shall inherit — the ascending line is searched by proximity, and the nearest degree found takes to the exclusion of everyone above it. Only once that degree is identified does the article split the estate between the two sides of the family: should they be of different lines but of equal degree, one-half shall go to the paternal and the other half to the maternal ascendants. The halving rule applies within a degree, never across degrees.
Per capita inside each half
Within a line the division is by head: in each line the division shall be made per capita. So with all four grandparents living, each takes a quarter — two of them sharing the paternal half, two the maternal. But the halves themselves do not move with the numbers. If both paternal grandparents survive and only one maternal grandmother does, the two paternal grandparents take a quarter each and the surviving grandmother takes the whole maternal half. A branch with fewer people in it is not disadvantaged; each person in it simply receives more.
An unequal degree defeats the split entirely
This is the trap in the provision. The half-and-half rule applies only to ascendants of equal degree. If one paternal grandfather survives while on the maternal side only a great-grandmother is living, they are not of equal degree — the grandfather is nearer — and the first sentence takes over. He inherits, and the more remote maternal ascendant takes nothing at all. Families expect a more remote relative to receive a reduced share rather than nothing, which is not how the ascending line works. Establish each survivor's degree before assuming the estate divides in two.
When this article is reached at all
None of it applies unless the enquiry has already got this far. The ascending line is reached only where the deceased left no descendants, so children and grandchildren have to be ruled out first, and the article itself begins in default of the father and mother, so a surviving parent takes ahead of every grandparent. A surviving spouse has separate rights that this provision does not address. Build the family tree with civil registry records and fix the dates of death on it; in an estate of this kind the tree decides the outcome more than any argument will.