• Haskell

interweave :: [a] -> [a] -> [a]   -- optional signature
interweave [] ys = ys
interweave xs [] = xs
interweave (x:xs) (y:ys) = x : y : interweave xs ys

main = mapM_ print $ interweave items1 items2

• Haskell

mapM_ print . concat 
	$ zipWith (\a b -> [a,b]) items1 items2

• Ruby

items1.zip(items2){|pair| puts pair}

• Python

a, b = iter(items1), iter(items2)
print(*zip_longest(a, b))

• Python

a, b = len(items1), len(items2)
for i in range(max(a, b)):
    if i < a: print(items1[i])
    if i < b: print(items2[i])

• Python

for pair in zip(item1, item2): print(pair)

• Python

print(*zip_longest(items1, items2))

• Clojure

(doseq [i (interleave items1 items2)]
  (println i))

• C#

for(int i = 0; i < Math.Max(items1.Count, items2.Count); i++)
{
  if (i < items1.Count) Console.WriteLine(items1[i]);
  if (i < items2.Count) Console.WriteLine(items2[i]);
}

• D

roundRobin(items1, items2).each!writeln;

• Fortran

do i=1, min(size(items1), size(items2))
  print *,items1(i), items2(i)
end do

• Go

for i := range min(len(items1), len(items2)) {
	fmt.Println(items1[i])
	fmt.Println(items2[i])

}

• Go

for i := 0; i < len(items1) || i < len(items2); i++ {
	if i < len(items1) {
		fmt.Println(items1[i])
	}
	if i < len(items2) {
		fmt.Println(items2[i])
	}
}

• JS

const shorter = _items1.length > _items2.length ? _items2 : _items1;
const longer = _items1.length <= _items2.length ? _items2 : _items1;
shorter.map((m, i) => {
  console.log(m);
  console.log(longer[i]);
});

• JS

const iterator1 = items1[Symbol.iterator]()
const iterator2 = items2[Symbol.iterator]()

let result1 = iterator1.next()
let result2 = iterator2.next()

while(!(result1.done && result2.done)) {
  if (!result1.done) {
    console.log(result1.value)
    result1 = iterator1.next()
  }
  if (!result2.done) {
    console.log(result2.value)
    result2 = iterator2.next()
  }
}

• Java

Iterator<String> iter1 = items1.iterator();
Iterator<String> iter2 = items2.iterator();
while (iter1.hasNext() || iter2.hasNext()) {
	if (iter1.hasNext()) {
		System.out.println(iter1.next());
	}
	if (iter2.hasNext()) {
		System.out.println(iter2.next());
	}
}

• Java

IntStream.range(0, Math.max(items1.size(), items2.size()))
	.boxed()
	.flatMap(idx -> Stream.of(
		items1.size() > idx ? items1.get(idx) : null,
		items2.size() > idx ? items2.get(idx) : null
	))
	.filter(Objects::nonNull)
	.forEach(System.out::println);

• Java

int i, A, B, n = max(A = a.size(), B = b.size());
for (i = 0; i < n; ++i) {
    if (i < A) out.println(a.get(i));
    if (i < B) out.println(b.get(i));
}

• Java

Iterator<T> A = a.iterator(), B = b.iterator();
boolean x = A.hasNext(), y = B.hasNext();
while (x || y) {
    if (x) out.println(A.next());
    if (y) out.println(B.next());
    x = A.hasNext();
    y = B.hasNext();
}

• PHP

# php 5.3+
print array_reduce( array_map( null, $items1, $items2 ), function($c,$d){return $c.($c?"\n":'').implode("\n", $d); }, "")."\n" ;

#php 5.6+
print implode("\n", array_merge( ...array_map( null, $items1, $items2 )))."\n";

• Pascal

for i := 0 to Min(items1.Count-1,items2.Count-1) do 
  writeln(items1[i],', ',items2[i]);

• Perl

print for zip @items1, @items2;

• Rust

for pair in izip!(&items1, &items2) {
    println!("{}", pair.0);
    println!("{}", pair.1);
}

• Scheme

(map (lambda (x)
       (display x)
       (newline))
     (foldr append '()
            (map list items1 items2)))

• Smalltalk

items1 with: items2 do: [:item1 :item2 |
  Transcript 
	showln: item1;
	showln: item2].

• Smalltalk

stream1 := items1 readStream.
stream2 := items2 readStream.
[stream1 atEnd ifFalse: [Transcript showln: stream1 next].
 stream2 atEnd ifFalse: [Transcript showln: stream2 next]]
	doWhileFalse: [stream1 atEnd or: [stream2 atEnd]].